Sand blasting systems for cleaning surfaces, stone sculpturing, and other uses of high energy abrasive streams are well known. Such an apparatus usually includes a supply means for storage and delivery of abrasive granules or grit to a blasting gun and a delivery system for a fluid stream capable of picking up the grit and passing the composite mixture of grit and carrier fluid through a nozzle on the gun which is held by an operator. The operator must control the quality of the composite stream, the quantity of grit picked up for the blasting operation, and the direction of the blasting medium as it impinges upon the object being worked upon. The type of cleaning or removal action depends upon the kind of abrasive particles being used as grit, the flow characteristics of the composite stream, and the quantity of grit injected therein, all of which must be readily controlled by the operator at all times.
Several known systems adapted for this purpose are shown in U.S. patents to Bemis, U.S. Pat. Nos. 3,828,478 of Aug. 13, l974, and to Wemmer 3,922,817 Dec. 2, 1975. Those constructions provide a supply means for dribbling abrasive grit from a reservoir means into a feed line connected to the blasting gun. The grit is delivered along the feed line to the nozzle of the gun by inducing a suction at the nozzle end of the grit feed line which draws a stream of air into the opposite end of the feed line behind the grit to carry it to the nozzle. Control of the degree of suction induced in the feed line causes more or less of the abrasive grit to be delivered to the nozzle and means are provided under the control of the operator to adjust the suction. Typically, a high speed working fluid such as steam or high pressure water is made to flow through a venturi where the grit feed line enters the nozzle assembly in order to induce a suction in the feed line. Air inlets with conventional valves have been positioned between the grit supply means and the feed line outlet to bleed more or less air into the grit feed line to effect the desired control of abrasive grit delivered to the nozzle as shown in Wemmer.